Contents

Idea

To make an smd reflow oven from an old sandwich oven.

Progress

April 17, 2014

Another run(see repo) confirmed that the overshoot is now fixed. We still have to think about the cooldown stage. In this run I opened the oven a little just after reaching the top. And then more and more towards the end. This makes the temparature decline more or less linear. I think that we need to test if the decline exceeds 5'C/s if we just open the thing after reflowing. If that is acceptable we can just buzz the buzzer at that point.

There seem to be problems with the display. Sometimes it starts showing all kinds of strange symbols. Once started it won't turn back to normal until the arduino is reset.
I also had issues with strange temperature readings but it turned out that one of the wires to the max chip was abit loose. Reattaching it firmly solved all issues there.

April 8, 2014

I assambled the thing and did the first full run. I visualized the heat-profile of this run in a fancy graph(see image in git repo).

What can be seen from this profile is that the oven is clearly capable of reaching the right temparature (which I was afraid would be a problem).

Secondly, the temperature change is not very steep. 1.5 degrees per second at the very max. When reading into the details of the reflow process it becomes clear that the strange shape of the ideal temperature curve is the result of different maximum dT/dt in the different phases of the process. Since we won't reach the maximum temperature change in many areas of the profile, our program will more or less boil down to: heat to 235 as quick as you can; then cool down as quick as you can. The only exception is the "soaking" stage where there is a maximum incline of 1 degree per second(violated in the first run:)).

Thirdly, there is a massive temperature overshoot. I had calculated for an overshoot of 5'C degrees and stopped the heating at 230'C.
The temperature still reached almost 260'C which is 25'C above the desired maximum. I have changed the code for the second run to stop heating at 205'C which should result in a maximum temperature of 235'C.

Some images of the assambled parts:

Note that not everything is connected at this stage: the buzzer is still missing and I connected only one of the buttons (I have to admin that I lost the other button:-().

March 12, 2014

Work has started on all fronts:

software: a beginning has been made

low voltage hardware: arduino and max6675 now have pin headers

high voltage hardware: a protective clip for the relay terminals was printed, it fits perfectly! The toster oven didassambly turned out harder than expected. Maybe we need another toaster oven:s This one has internal grill lines that will touch the pcb.